Variscan intra-orogenic extensional tectonics in the Ossa-Morena Zone (Évora-Aracena-Lora del Rı́o metamorphic belt, SW Iberian Massif): SHRIMP zircon U-Th-Pb geochronology

Abstract

Following a Middle–Late Devonian (c. 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism (c. 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician (c. 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345–335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean (c. 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

The multistage crystallization of zircon in calc-alkaline granitoids: U–Pb age constraints on the timing of Variscan tectonic activity in SW Iberia

“…Therefore, the use of presence versus absence of deformational features as the only criterion to establish a relative age between granitoids may lead to erroneous interpretations, such as the classification of unfoliated granitoids as 'late-orogenic', as is often the case in the Geological Map of Portugal, scale 1:500,000; (3) the ca. 336-335 Ma ages obtained for Fg and Pg coincide with the age interval Romeo et al 2006;Pereira et al 2009Pereira et al , 2012aCambeses et al 2014;Gladney et al 2014) obtained for the calc-alkaline intermediate granitoids of the Évora Massif (ca. 337-336 Ma) for a tonalite and mafic microgranular enclave of the Hospitais pluton (Moita et al 2013), and ca.…”

“…1). Variscan extensional tectonics is associated with the growth of high-to medium-temperature and medium-to low-pressure mineral assemblages related to isobarically cooled P-T paths along major shear zones (Chichorro 2006;Pereira et al 2009). The available geochronological data show that mylonitization in the Variscan shear zones took place in the period ca.…”

“…6): (1) the crystallization ages of the Beja gabbros and diorites Azor et al 2008); (2) the ages of metamorphism estimated in the range ca. 342-336 Ma of the Ventosa and São Brissos amphibolites and the Alcáçovas gneiss (Dallmeyer et al 1993;Pereira et al 2009); and 3) the cooling ages of ca. 339-337 Ma of the Beja gabbros (amphibole, Ar-Ar; Dallmeyer et al 1993).…”

“…Deformation of the ductile basement under high-temperature metamorphic conditions promotes partial melting and the formation of syn-kinematic crustal-derived magmas that interact with magmas of mantle origin. There is reason to argue that as a consequence of the Variscan oblique convergence, SW Iberia developed into a wrenching regime with the significant spread and formation of extensive calc-alkaline magmatism on both sides of the Rheic suture, inferred to have been emplaced after the Rheic Ocean had already closed, whose origin is not well understood (de la Rosa and Castro 2004;Romeo et al 2006;Jesus et al 2007;Pereira et al , 2009Pereira et al , 2012aMoita et al 2009Moita et al , 2013Rosa et al 2008;Braid et al 2010;Cambeses et al 2014;Gladney et al 2014).…”

Exhuming a cold case: The early granodiorites of the northwest Iberian Variscan belt—A Visean magmatic flare-up?

“…This early Carboniferous suite is widely represented in other areas of the WEVB (e.g., Schaltegger, 1997Schaltegger, , 2000Schulmann et al, 2002;Eichhorn et al, 2000;Cartannaz et al, 2007;Li et al, 2014;Mezger and Gerdes, 2016;Tabaud et al, 2015;Galán et al, 1997;Ledru et al, 2001;Rossi et al, 2009;Kubínová et al, 2017;Laurent et al, 2017;Žák et al, 2017), including the Ossa Morena zone of southwest Iberia (e.g., Dallmeyer et al, 1995;Ordóñez Casado, 1998;Montero et al, 2000;Ordóñez Casado et al, 2008;Romeo et al, 2006;Jesus et al, 2007;Pin et al, 2008;Pereira et al, 2009Pereira et al, , 2015Braid et al, 2012;Dupuis et al, 2014;Gladney et al, 2014;Cambeses et al, 2015). In the Ossa Morena zone rocks belonging to this suite are the volumetrically dominant granitoids, whereas in northwest Iberia the volumetrically dominant granitoids are both the previously described syntectonic leucogranites and the postorogenic granites (Fig.…”